Boring tools are known in the art in which a tool head is provided with a slot extending transverse to the axis of the tool head and in which a pair of opposite cutting knives are arranged. Such a boring tool is for instance disclosed in the "Schriftenreihe Feinbearbeitung: Feinbohren und Feinbearbeitung von Bohrungen" January, 1958, page 28. The boring tool disclosed therein has opposite cutting knives mounted in fixed position which therefore have to be removed from the tool head after the bore has been machined to avoid markings on the bore surface during retracting of the boring tool.
The same article discloses on page 43 another boring tool in which a pair of opposite cutting tools may be moved toward and away from each other by an oppositely threaded pin engaged into correspondingly threaded bores in the tool cutting knives. Thereby the cutting knives may be adjusted easily to any desired tolerance of the bore to be finished, but also in this construction the two cutting knives have to be removed from the tool head before the latter is retracted in axial direction of the finished bore in order to avoid markings during withdrawing of the tool head from the bore.
Such boring tools may also be combined with other tools, for instance with rolling tools, in order to finish cylinder bores in one operation. Such a combined tool is for instance disclosed in the U.S. Pat. No. 3,973,319. In this construction a rolling tool is arranged behind the cutting tool and the cutting tool carries out a rough finishing of the inner surface of the bore whereas the following rolling tool produces during the same operation a fine surface quality of the bore surface. In this construction too, the boring tool has to be removed from the tool head in order to assure that the finely finished surface is not damaged during retraction of the combined tool.
Such a necessary removal of the cutting tool is disagreeable, since, on the one hand, it prevents automatization of the tool in for instance a deep boring machine and, on the other hand, it results in a time loss during manual operation since the tool spindle in deep boring machines has to be brought at the end of the boring operation to a suitable position permitting withdrawal of the cutting knives. Furthermore, such an operation is also disagreeable for the operator since the boring tool is covered with oil.
The DT OS No. 22 23 696 discloses a boring tool in which a pair of opposite knives, guided in a slot transverse to the axis of the boring tool, may be moved in radially outward direction by a cone movable between the inner ends of the knives and this adjustment must also be carried out by hand so that an automatic operating cycle has to be interrupted for manually adjusting the knives. In this construction, too, the knives have to be retracted by adjusting the position of the cone in order to avoid damage to the finished bore surface during withdrawal of the boring tool and the readjustment of the knives to the necessary working diameter has to be carried out very carefully in order to provide the same result during finishing of a second bore.